Modulation is the fundamental process in any communication system. It is a process to impress a message (voice, image, data, etc.) on to a carrier wave for transmission. A band-limited range of frequencies that comprise the message (baseband) is translated to a higher range of frequencies. The band-limited message is preserved, i.e., every frequency in that message is scaled by a constant value. The three key parameters of a carrier wave are its amplitude, its phase and its frequency, all of which can be modified in accordance with an information signal to obtain the modulated signal.
There are various shapes and forms of modulators. For example conventional Amplitude Modulation uses a number of different techniques for modulating the amplitude of the carrier in accordance with the information signal. These techniques have been described in detail in “Modern Analog and Digital Communication Systems” by B. P. Lathi. Similarly conventional Frequency/Phase Modulation uses a number of different methods described in a number of textbooks. In all these techniques, carrier (which is a high frequency sinusoidal signal) characteristics (either amplitude, frequency, phase or combination of these) are changed in accordance with the data (or information signal). Thus there have been two major components of a modulated signal. One is the information-carrying signal and the other is the high frequency carrier.
Communication systems that have emerged in recent years include mono-pulse and Ultra-Wide Band communication systems. The problem with these systems is that all mono-pulse or Ultra-Wide Band communications systems form Power Spectrum Densities that tend to span very wide swaths of the radio spectrum. For instance the FCC has conditionally allowed limited power use of UWB from 3.2 GHz to 10 GHz. These systems must make use of very wide sections of radio spectrum because the transmit power in any narrow section of the spectrum is very low. Generally any 4 KHz section of the affected spectrum will contain no more than −42 dbm of UWB spectral power. Correlating receivers are used to “gather” such very wide spectral power and concentrate it into detectable pulses. Interfering signals are problematic. Since the communication system is receiving energy over a very wide spectrum, any interfering signal in that spectrum must be tolerated and mitigated within the receiver. Many schemes exist to mitigate the interference. Some of these include selective blocking of certain sections of spectrum so as not to hear the interferer, OFDM schemes that send redundant copies of the information in the hope that at least one copy will get through interference, and other more exotic schemes that require sophisticated DSP algorithms to perform advanced filtering. In addition, UWB systems have somewhat of a “bad reputation” because they at least have the potential to cause interference. A heated discourse has gone on for years over the potential that UWB systems can cause interference to legacy spectrum users.
Tri-State Integer Cycle Modulation (TICM) and other Integer Cycle Modulation techniques, which have now become known by its commercial designation, xMax, were designed by xG Technology, Inc., the Assignee of this application to help alleviate this massive and growing problem. Its signal characteristics are such that absolute minimal sideband energy is generated during modulation but that its power spectrum density is quite wide relative to the information rate applied. Also, a narrower section of the power spectrum output can be used to represent the same information. The method of determining timeslot duration in a MAC protocol described in this disclosure is applicable to any VOIP system but is described in the preferred embodiment below as applied to these types of integer cycle and pulse modulation systems in VOIP networks.